Tarek Abdoun

The failure of flood-control infrastructure, such as levees, due to a natural or manmade hazard can have significant repercussions, sometimes with dramatic and unanticipated consequences on human life, property, and the country’s economy. Thus, the integrity of a levee is an important issue that needs to be checked frequently. This can be achieved by long-term monitoring of the subsidence of the levee to identify potential weak sections and prioritize maintenance efforts. This paper presents the use of affordable sensing technologies, such as satellite-based interferometric synthetic aperture radar (InSAR), to monitor the settlement of the New Orleans area. Data from TerraSAR-X, the German Space Agency’s X-band satellite, was processed with the interferometric point target analysis technique in the gamma remote sensing software for widespread settlement monitoring. The time frame of the data is from 2009 to 2014. In situ measurements from GPS and extensometers are used to verify the remote sensing results, and a good agreement of displacement trends between the two methods was obtained. The favorable comparison increases confidence in the suitability of remote sensing techniques to monitor the long-term displacement of entire levee systems.

Shear wave velocity is an essential parameter to characterize the dynamic behavior of the soil in small-scale samples, centrifuge models as well as field deposits. In this paper, the bender elements system in the centrifuge lab at Rensselaer Polytechnic Institute is used to characterize the shear wave velocity and the small strain shear modulus of dry and saturated Ottawa sand. It was found that the shear wave velocity of saturated sand is smaller than that of dry sand having the same relative density. Also, the paper describes an analytical system identification (SI) method that can be used in the absence of bender elements to estimate shear wave velocity from acceleration recordings. The SI and bender elements were found to be in very good agreement with each other confirming the validity of both techniques, which are fully independent.

The paper presents a centrifuge test of a level site consisting of granular soil deposits and an embedded structure subjected to various bi-axial base excitations. The test was conducted at RPI NEES centrifuge facility to assess the dynamic response characteristics of soil-structure interaction (SSI) under multidimensional conditions. Synthetic sinusoidal waves were used as base excitations to test dense model under biaxial shaking. A dense array of accelerometers was used to monitor the deposit response (beneath and outside of the structure, along with pore water pressure transducers. The observed acceleration and pore pressure are used along with non-parametric identification procedures to estimate the corresponding dynamic shear stress-strain histories. The measured results along with the obtained histories are used for two purposes. First, to shed light on the mechanisms of soil-structure interaction under biaxial shaking. Secondly, to show the difference in soil behavior at different locations beneath the footing when subjected to biaxial shaking. The later objective is evident when comparing soil stress-strain loops and dilation behavior at locations beneath the structure, and those measured in the free field.

The integrity and reliability of levees are essential components of homeland safety. The failure of such systems due to a natural or manmade hazard can have monumental repercussions, sometimes with dramatic consequences on human life, property and the country’s economy. This paper presents some results of integrated monitoring and modeling to assess the performance-based response of a levee section. The modeled levee is part of the Whale’s Mouth section on Sherman Island where satellite images and in-ground GPS sensors are used for displacement measurements. The Whale’s Mouth levee is modeled using the large deformation option of the finite element program PLAXIS 2D. The model is used to establish a deterministic performance response under maximum water level loading and to investigate the effect of peat decomposition on the deformation response of the levee section. The remote sensing and in situ data are used to calibrate the numerical model. The results are compared to the pre-defined limit state and illustrate how the peat layer decomposition affects the modeled levee section performance. The concept of performance limit states of these critical structures provides a means to quantitatively assess the functionality of an earth structure under severe storm loading events. The probability of exceeding a prescribed limit state is defined based on the strain or gradient levels in potential emerging failure zones. The variation in strength properties and hydraulic conductivity of the levee embankment, as well as the rate of rising water level and duration of flooding, may lead to the progression of the structure state from a low probability of exceeding adequate functionality to the probability of exceeding a high limit state, i.e., imminent failure. The displacement data collected during these loading and unloading events is used to establish the levee condition assessment on the basis of the performance limit states.

In the first part of this research, centrifuge model testing was used to investigate the relationship between the coefficient of lateral earth pressure at rest, K₀, and overconsolidation ratio, OCR, for clean sand for the whole loading-unloading-reloading cycle. Tactile pressure sensors were used to measure the change in K₀ in two saturated centrifuge models conducted on Ottawa sand_. A relationship was developed between the change in K₀ and OCR both for unloading and reloading conditions. In the second part of this research, the same tactile sensor was used to test the influence of extensive liquefaction on K_0. A centrifuge model was subjected to very strong shaking causing extensive liquefaction. Lateral earth pressure was measured before and after liquefaction. It was found that there is a significant reduction on K₀ due to extensive liquefaction which explains the increased vulnerability of sandy deposits after liquefaction, as observed by many researchers.

The estimation of liquefaction potential of soil deposits is mainly based on field liquefaction triggering charts based on the Seed and Idriss Simplified Method. Currently, these field liquefaction triggering charts seem to be generally conservative, as they do not take into account factors that tend to increase the liquefaction resistance of natural sand deposits located in seismic areas. The work presented in this paper incorporates data from the field, as well as centrifuge experiments performed at RPI. The experimental work involved simulating several decades to a century of earthquake events applied to a 6-m uniform silty sand deposit. The results show that repeated earthquake shaking generally has the effect of increasing the liquefaction resistance of soil deposits over time. This increase in liquefaction resistance does not seem to be fully accounted for by the simultaneous increase in the shear wave velocity of the soil. In other words, sites that have been subjected to hundreds of small earthquakes as well as some larger liquefying events may not liquefy again unless they are subjected to very intense earthquake shaking.

T-walls are floodwalls that consist of reinforced concrete in an inverted “T” shape. These structures are supported on batter piles, and they are commonly used in New Orleans and the surrounding area to protect low-lying regions against hurricane-induced flooding. Since the soils in southern Louisiana often include very soft and compressible clays, T-walls need to be designed to resist the stresses and bending moments that can develop in the batter piles due to settlements. However, the current T-wall design procedure for estimating settlement-induced bending moments is largely based on numerical models that have only been validated to a limited extent. Therefore, a series of centrifuge tests is currently being performed to investigate and physically model the effects of settlement-induced bending moments on pile-supported T-walls. This paper describes one of these centrifuge tests as well as the development and validation of a corresponding numerical model. The results from the numerical model are in good agreement with the centrifuge test results, which is an indication that numerical modeling can be used to realistically estimate the magnitude and distribution of settlement-induced bending moments.